Newbie Question, Need one more gear....

[bmwhitt]

I searched but couldnt find exactly what I needed. I recently bought a 1964 GMC 3/4 ton with the 305 V6. I have been driving the wheels off and love every minute of it. It is lwb and has a utility bed on it. My question is, what is a direct replacement for the rear end, gears, or maybe a 5 speed trans that bolts to the big V6? The motor seems to be screaming at about 45 mph or so...( my speedo isnt working). This was my grandad's truck and dad said it would pull a house down if you could keep the a$$ end down. I just want it to be a little more drivable maybe 65 mph or so.....Thanks ahead of time and sorry if this is a repeater!!

[spacedebris]

Not exactly cheap, but there is always the gear vendor OD option. My grandfather had an 85 ford F250 that was the same way. 50 MPH and it was out of steam with the engine screaming near redline. Added the GearVendor and it would cruise just fine at 65-70MPH.

[bmwhitt]

Thanks for the reply. There has to be an easier way...maybe a direct swap rear end from a late model or something like that.

[FetchMeAPepsi]

Your screaming is probably closer to 50 - 55mph. That's the norm.
As for getting her to go faster, well, you can swap out the rear end gears to a 3.73 or so, but remember that you're pushing 4k lbs of thick unyielding steel. These old farm trucks weren't made to go that fast b/c the reaction time in them is crap. They become a missle.

(and who needs to haul hay and get it there RIGHT NOW, right? lol!)

Anyway, if you really want you can swap out those rear end gears and you'll be fine. You might consider bigger tires too. What kinda rear end do you have now?

[kieth]

Before changing the trans/rear end pm me your spec's. I will send you a detailed performance spreadsheet. Kieth

I need engine
transmission
rear ratio
tire diameter(ground to top of rear tire)

I will send you a excell spreadsheet that shows usable torque to the ground and speeds at 1500, 2000, 3000, 4000 rpm........

[factorystock]

Originally Posted by bmwhitt

My question is, what is a direct replacement for the rear end, gears,

GMC 3/4 tons with leaf springs came with either 4.10 or 4.56 Dana 60 axles. Direct replacement would be a Dana 60 with 3.54. This axle can be found in big block 69 thru 72 GMC 3/4 ton with leaf springs. THis would be a do it yourself job. To replace the gears in your axle would require you to take your vehicle to a professional rear end shop and buy a 3.54 ring and pinion set and a carrier, and have the shop do it, which may be more expensive than finding a whole rear axle drum to drum in a wrecking yard.

[bmwhitt]

Thanks for the help. Ill get all the info when I can. Appreciate it!
Posted via Mobile Device

[sdcfish]

I just dealt with the same ****......I have a 65 C-20 with a 4 speed that had an Eaton HO-52 rear end....check that out on google and see the images to match it up. I am guessing you have the same **** I did which was that HO-52 rear end with 4.57 gears. I couldn't get over 45 - 50 mph either. 410 gears where the best I could do in that rear end. Here are your options:
1. Buy a new 14 bolt rear end with a 3.73 or lower gear and pay $1500 or so to exchange, plus the cost of a used rear end.
2. Change the gear in the diff to a 410 which is the best you can do in that Eaton.
3. Try to find an aftermarket gear which I was lucky to have done, and it's a 390 gear...which I now can run 62-63 mph comfortably.
4. Go for a 410 gear change, and get a new transmission, like a NV4500 with an overdrive....you can achieve .75 x 410 to get your overdrive running gear number.
5. sell that truck and buy a new one that has already been swapped out on this issue

Good luck...that's what I got for ya. Just been there done that.

[sdcfish]

Oh...I paid $750 for the 3.90 gear exchange.....but it's gonna be hard to find another one....I got really lucky A 4.10 gear exchange is more like $600.00 all done. Maybe with a bigger tire too it could get your more like 55-60 mph My rims are 18" and my tire is a 255/65/18. Not that high a profile tire...just medium in my opinion.

[macdodd]

I have a '69 c-10,,,3 speed auto trans in it,,,need to change out the rear end gears,,,,gas milage in bad,,,LOL Taking it to a shop and have them check the rear end gears and we'll go from there. At 2200 rpms,,,I'm running around 50mph. Its a daily driver and we want to cruise up the mountains in the fall,,,,,just don't want to burn up the rear end or trans. Its a 350/305 heads, with mild cam. Edlebrock carb,,,

[Figuarus]

A gear change will only do so much for you, but the best way of getting any kind of upgrade is to go to a 700R4 trans.

I did the math, and with my TH400 and 4.11 rear end, I'm screaming at 3300 rpm @ 65 MPH.

With a 700R4, and keeping my 4.11 gears, The OD .70 final gear will allow me to travel at about 2200 rpm. That's a change of about 35%.

If you do gears only, Say from 4.11 to 3.73, you will only see a decrease of about 10% in RPM @ 65 MPH.


What transmission do you have, and what gear ratio are you planning on changing to?

[frankslagoon]

I don't know what's available in automatics for that V-6. anybody know what the bolt pattern is? if you could find one of those 3:54 complete rearends. that would work good and you could do the swap in the driveway.

[Figuarus]

Originally Posted by frankslagoon

I don't know what's available in automatics for that V-6. anybody know what the bolt pattern is? if you could find one of those 3:54 complete rearends. that would work good and you could do the swap in the driveway.

V-6? Sounded like he stated 350 with 305 heads on it...

[KALAHEO]

Depending on how big of gear change he may need new carrier

[TJ's Chevy]

3.73 gear and 31 inch tire will be a nice change. Far as I know the GMC 305 V6 bell housing should be the same bolt pattern as a SBC or a BBC because they used the SM-420, an NV4500 would be a good swap or even a richmond or tremic. There's lots of choices out there. Or like mentioned a gear vendors od would work but is almost as spendy as a good used 5 speed. The tremic and richmond are both $2K trannys. 3.73 and a 31 inch tire will be at 2300 rpm at 55 miles an hour. With 4.11's its 2500 and 4.56's it 2750. Sounds like you have 4.56's but remember...the 305 is not an rpm machine by any means...2500 rpm will make this thing sound like its tached out. My 64 GMC has 4.11's I believe and I travel at 55 and it sounds tached out, but cruise at 45 and its super quiet...I can shift from 2nd to 3rd at 1500 and lug it and it loves it...these were made to be lugged...when I lug it I can feel the torque but when I wined it up its a waste of time. Which you've probably noticed. lol

[patrickk1]

In regards to the BIG v6, You need a special adapter to use an automatic. Either a factory original automatic adapter(hard to find but can be found) or new (they sell for about $1k). The BIG v6's had their own special bolt patterns. To use the nv4500 on the v6, it might be cheaper to buy a 68+ 305V6 (or bigger) manual bellhousing (has 5 1/8 bore) and buy the GM pattern to Dodge NV4500 plate adapter from Advanved adapters (~$285).

[Figuarus]

Am I missing something? I don't see where mcdodd made any reference to having a V6.

I read that as 350 WITH 305 heads. What did I miss?

[patrickk1]

He didn't mention v6 but he did bring back an almost 3 year old thread instead of starting his own. I figured I might as well add some information to the previous posts that were v6 specific.